The service VoLTE (for Voice over Long Term Evolution) is a communications service of the voice type provided over a mobile communications network from a 4G access network. A mobile communications network allowing a VoLTE communications service to be provided is for example based on a conventional infrastructure of a circuit telephony network and of 2G/3G/4G data network (data). The core of the communications network or sub-system of the communications network is for example based on an IMS (for IP—Internet Protocol—Multimedia Subsystem) and an IP EPC (for Internet Protocol Evolved Packet Core) architecture. The VoLTE service is for example defined in the standard GSMA IR92—IMS Profile for Voice and SMS, Version 7.0, 3 Mar. 2013.
Such architectures and infrastructures are also used to provide other communications services to the terminals connected to the mobile network. For example, an enhanced communications service of the RCS (Rich Communication Suite) type may be provided. The RCS service is for example defined in the GSMA standard RCC61, RCS Common Core Service Description Document, version 1.0, 16 Sep. 2014.
In order for a terminal to be able to support a communications service of the VoLTE or RCS type, it is necessary for the terminal to dispose of a protocol stacking mechanism, otherwise known as an IMS SIP stack when the protocol used is the SIP (for Session Initiation Protocol) protocol. Such a protocol stack allows the terminal to communicate with the sub-system of the IMS mobile communications network. The IMS SIP stack of the terminal allows the terminal to register in the sub-system of the IMS mobile communications network via the SIP (for Session Initiation Protocol) protocol, then to receive and/or to transmit communications according to a communications service based on an IMS architecture. Such communications services are for example the VoLTE, RCS, ViLTE communications services (for Video over LTE), VoWIFI (for Voice over WIFI).
In order to save the battery life of the terminal and to reduce the number of reporting messages within the mobile communications network, a single IMS SIP stack may be used in the terminal in order to provide several communications services based on the IMS architecture. Thus, for example, for a terminal designed to implement a VoLTE communications service and an RCS communications service, the reporting streams for VoLTE or RCS communications will be transmitted according to the SIP protocol to the IMS sub-system, the media streams for a VoLTE communication will be transmitted according to the RTP/RTCP protocol and the media streams for an RCS communication of the messaging type will be transmitted according to the MSRP protocol.
It is assumed that the communications network of an operator to which a user is subscribed implements such a mechanism according to which only one IMS SIP stack is needed to provide several communications services based on the IMS architecture. The communications network of the operator to which a user is subscribed is commonly called nominal network or HOME network. When a user turns on his/her terminal, the terminal automatically connects to his/her HOME communications network when an access point to this network is accessible for the terminal.
When the user changes location, for example when he/she changes country, the HOME communications network might no longer be available if the operator to which the user is subscribed has not deployed any infrastructures within this country.
In this case, in general, the operator to which the user is subscribed has negotiated “roaming” agreements with one or more other operator(s) who dispose(s) of communications network infrastructures within the country in which the user is located. Such a communications network is subsequently referred to as a visited network. When a terminal is connected to a visited network, the terminal is said to be in a “roaming out” situation.
Such agreements allow interconnections between the visited communications network and the nominal communications network to be physically established. Thus, when the terminal of the user can connect to the visited communications network, it may access communications services provided by the visited communications network or the nominal communications network.
The services accessible by the terminal of the user when the terminal is connected to a visited communications network depend on the interconnections established, and hence on the agreements negotiated between the two operators. Thus, for example, it is possible for an interconnection to be established between the visited communications network and the nominal communications network in order to provide a VoLTE communications service to the terminal of the user when this terminal is connected to the visited communications network.
However, it is also possible for an interconnection not to be established between the visited communications network and the nominal communications network in order to provide an RCS communications service. For example, the absence of such an interconnection may be caused by the absence of roaming agreements between the two operators for this communications service, or because the visited communications network is not able to provide the RCS communications service, for example since the visited communications network is lacking network resources dedicated to the service in question.
Thus, when the terminal of a user is connected to a visited communications network, the user suffers a loss of quality of experience since the user then no longer has access to services, for example the RCS service in our example, to which he/she usually has access when his/her terminal is connected to the nominal communications network.